The present invention relates to the preparation of methyl chloroformate, a composition of known utility as an intermediate in the manufacture of pharmaceuticals, agrichemicals, plastics and other organic compositions.
Methyl chloroformate has a molecular weight of 94.50 and a boiling point of 71.4.degree. C. Its structure is characterized roughly as follows: ##STR1## This structure is believed to account for the relative stability of the composition.
Methyl chloroformate ("MCF") is formed by the reaction of methanol with phosgene. Heretofore, this has been carried out in the gaseous phase in a counter-current reactor packed with ceramic or activated carbon. Purities achieved by this method have not been good: about 85% is as good as can be expected. The major contaminants are dimethyl carbonate ("DMC"), methyl chloride, water, HCl, and phosgene (BP=8.02.degree. C.) Heretofore, production of a high purity product has required distillation of this crude product.
Methyl alcohol, even if in excess of phosgene, can be "flashed" or distilled off and therefore is not a serious problem per se. However, excess methyl alcohol reacts with MCF, and the result is DMC. For this reason, the reaction should be run with an excess of phosgene. A second way DMC can be formed is by running the MCF reaction too hot. DMC can also be formed by reaction of excess methyl alcohol with MCF in the still, while the methyl alcohol is being flashed off. Since the DMC reaction is slow, this leaves little to worry about unless the still is left on total reflux for a long period of time. For every 1 wt.-% of methyl alcohol that reacts with MCF in a closed system, 2.95 wt.-% loss in the purity of MCF occurs, 2.81 wt.% gain in DMC occurs, and 1.14 wt.-% gain in HCl occurs.
If the MCF is above 53.degree. C. HCl gas will be evolved because of limited solubility. In that case, the temperature would have to be lower for HCl gas not to evolve. For example: if a MCF solution containing 99% MCF and 1% methyl alcohol totally reacts due to time, temperature, or a combination, the ultimate result would be 96.05% MCF, 2.81% DMC, and 1.14% HCl or about 3% MCF purity loss.
Hydrogen chloride is another of the lesser contaminants of MCF. It is soluble in the range of 0.10 wt.-% at 71.degree. C. to 3.25 wt.-% at 15.degree. C. in MCF. As a by-product of the MCF preparation, HCl is almost impossible to completely eliminate and is at this time believed to be responsible for two problems. The first is the loss of MCF as a vapor along with the exiting HCl gas. This vaporizing effect due to the low vapor pressure of HCl lowers the MCF yield by as much as 20%, if sufficient cooling is not applied to the gas stream. This yield is based on the lowest concentration of raw material used, which is methyl alcohol. The second problem is the reaction of HCl gas with unreacted methyl alcohol in the MCF production. This is a catalytic effect accelerated by use of ceramic (clay) packing in the reaction column. The end products of this unwanted side reaction are methyl chloride and water. Methyl chloride is a gas at 15.degree. C., evolves mixed with HCl gas.
Water is insoluble in MCF and vice versa. It is somewhat difficult to separate from MCF, but rapid distillation (flash-off) with some total reflux followed by total take-off will usually rid the MCF of most of the water and a good part of the excess methyl alcohol. MCF is decomposed by water and under certain conditions the water either disappears from sight or is evenly dispersed throughout the MCF. With time, if it doesn't all react, it will float to the top of the MCF. The purity of good MCF can drop by 1% in less than a week due to contact with a slight amount of water. Pure distilled water has been used to quickly wash MCF and then the pH of the wash water noted immediately. The pH was always 1 or less for each successive wash of the 99% pure MCF, using fresh water each time.
Phosgene should be no problem in that the "flash-off" should quickly and easily remove it. The problem is the reuse or destruction of the exiting phosgene gas from the reaction loop, and distillation. This can be solved quite simply by scrubbing all the HCl, methyl chloride, and phosgene out with a mix of water, methyl alcohol, and caustic, but this is expensive.
Those skilled in the art need not be reminded that both raw materials and products in processes of this sort are both toxic and corrosive, and that very substantial care is required in both construction and operation thereof.